On Thu, Dec 21, 2017 at 12:38:12PM +0200, Alexey Dobriyan wrote:
> On 12/21/17, Eric W. Biederman <ebied...@xmission.com> wrote:
> > I have stared at this code, and written some test programs and I can't
> > see what is going on. alloc_pid by design and in implementation (as far
> > as I can see) is always single threaded when allocating the first pid
> > in a pid namespace. idr_init always initialized idr_next to 0.
> >
> > So how we can get past:
> >
> > if (unlikely(is_child_reaper(pid))) {
> > if (pid_ns_prepare_proc(ns)) {
> > disable_pid_allocation(ns);
> > goto out_free;
> > }
> > }
> >
> > with proc_mnt still set to NULL is a mystery to me.
> >
> > Is there any chance the idr code doesn't always return the lowest valid
> > free number? So init gets assigned something other than 1?
>
> Well, this theory is easy to test (attached).
I'll give this a shot and report back when I get to the office.
> There is a "valid" way to break the code via kernel.ns_last_pid:
> unshare+write+fork but the reproducer doesn't seem to use it (or it does?)
that sysctl is root only, so that isn't at play here.
Dav
